Field of the Invention
The present invention relates to an image forming apparatus that uses an electrophotographic process or the like.
Description of the Related Art
Hitherto, as an electrophotographic image forming apparatus, there is known an image forming apparatus having a configuration that uses an intermediate transfer member. In a primary transfer step, a toner image formed on a photosensitive drum serving as an image bearing member is transferred onto the intermediate transfer member (hereinafter referred to as “intermediate transfer belt”). After that, the primary transfer step is repeatedly executed for toner images formed in respective image forming stations for yellow (Y), magenta (M), cyan (C), and black (Bk), to thereby form toner images having a plurality of colors on the intermediate transfer belt. Subsequently, in a secondary transfer step, the toner images having the plurality of colors formed on the intermediate transfer belt are transferred onto a surface of a recording material such as paper fed from a sheet feeding unit. The toner images transferred onto the recording material are then fixed onto the recording material by a fixing unit, and a color image is formed thereon.
In such an image forming apparatus, a control unit receives print data from an external apparatus such as a host computer, and expands bitmap data based on the received print data. Then, after finishing expanding the bitmap data, the control unit outputs the expanded bitmap data to an engine unit configured to control image formation as video data. The control unit includes an expansion circuit for each image forming station controlled by the engine unit in order to transmit the video data to each image forming station. The control unit transmits the video data of a corresponding color to each image forming station in accordance with an image writing signal (hereinafter referred to as “/TOP signal”) output from the engine unit, to thereby perform the image formation.
Further, the image forming apparatus may be instructed to perform a printing operation for a sheet having an indefinite size by the external apparatus such as a host computer. When instructed to perform the printing operation for the indefinite size, the engine unit performs appropriate printing in accordance with the size of the sheet no matter which size (length of the sheet in a conveyance direction) the sheet set in the sheet feeding unit has. In such an image forming apparatus, when the size of the sheet placed in the sheet feeding unit is unknown, the image formation is performed by determining an image formation interval serving as a time interval at which the /TOP signal is output from the engine unit to the control unit with reference to the maximum size of the sheet to be placed in the sheet feeding unit. Then, the image formation is performed by changing the image formation interval in accordance with the size of the sheet so that a distance between the sheets becomes a predetermined interval at a timing at which the size of the sheet is determined. A proposal to suppress deterioration in productivity by improving throughput in this manner is disclosed in, for example, Japanese Patent Application Laid-Open No. 2008-122935.
According to the above-mentioned image forming apparatus, an output interval of the /TOP signal is determined based on the size of the sheet. Therefore, when the size of the image formed by the control unit (length of the image in the conveyance direction) is larger than the size of the sheet placed in the sheet feeding unit, there may be an overlap between periods during which video signals are output. That is, while the control unit is outputting the video signal of the image corresponding to a leading sheet, the /TOP signal corresponding to the following sheet may be received from the engine unit. As a result, there is a problem in that the printing operation cannot be normally performed due to the overlap between the periods during which the video signal of the image corresponding to the leading sheet and the video signal of the image corresponding to the following sheet are output.